Mutants blocked or altered at different steps in the synthesis, assembly, or secretion of Ig by mouse myeloma cells have been isolated and are being characterized. In addition, we have established methods for transfecting Ig genes into lymphoid cells and having them expressed. We are now using these methods to explore Ig gene and protein function. We have prepared a transfection vector that contains the entire MPC-11 gamma[unreadable]2b[unreadable] gene. Experiments have shown that, when this gene is transfected into lymphoma cells, the predominant mRNA made is of the membrane type; when we transfect this gene into myeloma cell, the predominant mRNA is of the secreted form. We have used this gene to identify sequences 3' of the gamma[unreadable]2b[unreadable] gene which are important for its polyadenylation and processing to the mRNA encoding the secreted immunoglobulin. We have also used gene transfection to construct and express novel Ig molecules. We have joined the variable region from the heavy chain to the light chain constant region and have produced light chain heterodimers which bind antigen. In addition the variable region from a mouse myeloma has been joined to human constant region genes. When these genes are transfected a chimeric protein is produced with the variable regions and specificity of the mouse myeloma joined to the human constant region with its associated effector functions. We also have reconstructed and expressed a heavy chain which is completely human. We are now beginning to construct chimeric molecules of novel structure and will investigate their biologic functon. The chimeric molecules have wide potential applicaion in diagnostics and therapy. (AB)